[PATCH RFC] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE accounting

[Li Zhijian <lizhijian@fujitsu.com>]

Goto-san reported confusing pgpromote statistics where
the pgpromote_success count significantly exceeded pgpromote_candidate.
The issue manifests under specific memory pressure conditions:
when top-tier memory (DRAM) is exhausted by memhog and allocation begins
in lower-tier memory (CXL). After terminating memhog, the stats show:

$ grep -e pgpromote /proc/vmstat
pgpromote_success 2579
pgpromote_candidate 1

This update increments PGPROMOTE_CANDIDATE within the free space branch
when a promotion decision is made, which may alter the mechanism of the
rate limit. Consequently, it becomes easier to reach the rate limit than
it was previously.

For example:
Rate Limit = 100 pages/sec
Scenario:
  T0: 90 free-space migrations
  T0+100ms: 20-page migration request

Before:
  Rate limit is *not* reached: 0 + 20 = 20 < 100
  PGPROMOTE_CANDIDATE: 20
After:
  Rate limit is reached: 90 + 20 = 110 > 100
  PGPROMOTE_CANDIDATE: 110


Reported-by: Yasunori Gotou (Fujitsu) <y-goto@fujitsu.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
---

This is markes as RFC because I am uncertain whether we originally
intended for this or if it was overlooked.

However, the current situation where pgpromote_candidate < pgpromote_success
is indeed confusing when interpreted literally.

Cc: Huang Ying <ying.huang@linux.alibaba.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb..4715cd4fa248 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1940,11 +1940,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 		struct pglist_data *pgdat;
 		unsigned long rate_limit;
 		unsigned int latency, th, def_th;
+		long nr = folio_nr_pages(folio)
 
 		pgdat = NODE_DATA(dst_nid);
 		if (pgdat_free_space_enough(pgdat)) {
 			/* workload changed, reset hot threshold */
 			pgdat->nbp_threshold = 0;
+			mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE, nr);
 			return true;
 		}
 
@@ -1958,8 +1960,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 		if (latency >= th)
 			return false;
 
-		return !numa_promotion_rate_limit(pgdat, rate_limit,
-						  folio_nr_pages(folio));
+		return !numa_promotion_rate_limit(pgdat, rate_limit, nr);
 	}
 
 	this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
-- 
2.43.5